Laminated materials and coatings are known from the prior art, comprising a substrate serving as a support coated by a film printed with a decoration. Typically, the decoration reproduces a noble and/or natural material. For example, a known laminate is formed by a substrate comprising a given number of superimposed glass fibre reinforced PVC films, and a printed film, also reinforced and therefore not extensible, or non reinforced and therefore extensible, reproducing the appearance of a noble material, such as wood.
This technique is commonly used for producing imitation wood floorings which are named “wood effect” or “parquet effect” or simply “laminated parquet”.
The development of the printing techniques, including digital printing, allows a very high quality printing capable of a very good imitation of any natural material. For example, it is possible to obtain accurate reproduction of the colours, the tones and the grains of any type of wood. However, the above is not enough to confer to the product an acceptable aspect, given that the surface of the coating film is glossy and smooth to touch, thus being absolutely unnatural and quite far from the appearance of a natural material such as wood.
In order to overcome this drawback, it is known to subject the laminate to an embossing process, after the coupling between the substrate and the printed film. The embossing process imparts a series of reliefs on the surface of the laminate, creating a roughness that is perceptible to touch, bringing the product closer to the desired natural aspect.
A first technique provides for imparting a continuous embossing, substantially by passing the laminate continuously between an engraved cylinder and an opposite pressure cylinder also referred to as “counter cylinder”. This technique is quick and can substantially be implemented directly “in line” with the printing and coupling between the substrate and printed film, operating at the same speed. However, this solution is not satisfactory because the embossing pattern generated by the engraved cylinder is repeated periodically at each complete rotation of the cylinder, thus being not coordinated with the printing. For example, the pattern of the roughness is not consistent with the grains, nodes, or other distinctive features of the wood reproduced by the printing. Substantially, the aspect of the finished product, although slightly improved to touch with a surface that seems rough instead of smooth, is clearly perceived as artificial.
In order to overcome also this drawback, processes of registered or synchronized embossing are also known, wherein the roughness produced by the embossing on the surface of the laminate are specifically coordinated with the printed decoration. It can be understood that the embossing member is specific for a given decoration in this case.
The process of registered embossing produces a realistic imitation of a natural material (wood, stone, etc. . . . ), but reveals some technical problems. Actually, it is necessary to ensure that the embossing is strictly carried out in a registered manner, so that the three-dimensional profile engraved onto the surface of the laminate accurately matches with the printing on the film. Actually, a possible misalignment or delay/advance of the engraved cylinder would jeopardize the process creating a product with unnatural and unsatisfactory appearance. In the prior art, in order to attain this object, the registered embossing is carried out in a discontinuous manner, i.e. substantially through the following steps:    a) a laminated sheet, produced continuously, is cut into panels having a predetermined dimension, for example 2×4 meters;    b) each panel is conveyed singularly to an embossing machine which substantially operates as a press; the panel is aligned on the two axes, with respect to the embossing member, with the help of suitable references;    c) finally, the embossing press engraves the desired relief (embossing) on the surface of the panel.
An accurate aligning, as indicated in step b), allows obtaining an accurate registered embossing. The references can be generated during the printing, for example in portions of the panel intended to be discarded during a subsequent step of cutting into strips.
A drawback of this technique lies however in the discontinuous operation, which is slow and expensive. Furthermore, the off-line embossing press is expensive and bulky. Since the referred laminate is typically a low cost product, there is a need of simplifying and making the process as inexpensive as possible. For such purpose, a felt need is to operate the embossing continuously, i.e. directly on the sheet (or strip) of laminate instead of on previously cut panels, but in the prior art, up to now, this has not been deemed viable.
One of the reasons is the high pressure that is required for the embossing, and which does not allow to use the registering techniques which are known from the field of printing.
More in detail, and with the aim of disclosing the technical problem, it is considered that the thrust between printing cylinders is generally less than 10 kg for each centimeter of length of the generatrix of the cylinder (kg/cm). Typically, the thrust is 5-10 kg/cm. This value makes possible a forward or rearward sliding of the cylinders, allowing keeping registered the various colours which are applied one after the other in the polychrome printing. However, in the case of the embossing, the thrust is considerably higher, typically greater than 20 kg/cm and preferably between 50 and 75 kg/cm. So far, a sliding of the engraved cylinder with the aim of maintaining the registered condition has been deemed incompatible with such thrust.